Abstract: When tank of oil or flammable gas is to be put into service, tank cleaning is usually needed, followed by displacing the iniside oxygon with inert gas, so as to prevent fire and explosion. Introduced are four displacing methods, namely complete mixing, non-mixing, vaccum deaeration, and pressure displacement. Different methods are compared in proof of calculations. The order of inert consumption going from the less to the more is vaccum deaeration, complete mixing and pressure displacement. Vaccum deaeration requires evacuation equipment and vessels which stand pressurization; The key to non-mixing method is to fill the vessel with water or other liquid so as to remove the original gas and improve displacing efficiency; Complete mixing is simplist in operation, but it has a mediate inert consumption. It shall be noticed that the formula for inert consumption are idealized, therefore they are only for reference.